The present invention is concerned with the characterization of epitaxial layers. The present invention is particularly useful in characterizing epitaxial layers of semiconductor alloys such as mercury cadmium telluride, lead tin telluride, indium arsenide antimonide, gallium arsenide phosphide, and others.
For the purposes of simplicity, the present invention will be described with reference to a particular semiconductor alloy: mercury cadmium telluride. The common chemical notation for mercury cadmium telluride, (Hg,Cd)Te, or Hg.sub.1-x Cd.sub.x Te, will be used.
Mercury cadmium telluride is an intrinsic photodetector material which consists of a mixture of cadmium telluride, a widegap semiconductor (E.sub.g = 1.6ev), with mercury telluride, which is a semimetal having a "negative energy gap" of about 0.3ev. The energy gap of the alloy varies linearly with x, the mole fraction of cadmium telluride in the alloy. By properly selecting "x", it is possible to obtain mercury cadmium telluride detector material having a peak response over a wide range of infrared wavelengths.
(Hg,Cd)Te is of particular importance as a detector material for the important 8 to 14 micron atmospheric transmission "window". Extrinsic photoconductor detectors, notably mercury doped germanium, have been available with high performance in the 8 to 14 micron wavelength interval. These extrinsic photoconductors, however, require very low operating temperatures (below 30.degree. K). (Hg,Cd)Te intrinsic photodetectors having a spectral cutoff of 14 microns, on the other hand, are capable of high performance at 77.degree. K.
At the present time, most (Hg,Cd)Te is produced by bulk growth techniques such as the technique described by P. W. Kruse et al. in U.S. Pat. No. 3,723,190. High quality (Hg,Cd)Te crystals are produced by this bulk growth technique.
Epitaxial growth techniques offer a number of potential advantages over bulk growth techniques. An epitaxial layer is a smooth continuous film grown on a substrate, such that the film crystal structure corresponds to and is determined by that of the substrate. The desired epitaxial layer is single crystal with uniform thickness and electrical property. The substrate has a different composition or electrical properties from that of the epitaxial layer.
A number of epitaxial growth techniques have been investigated in an attempt to grow (Hg,Cd)Te layers. Vapor phase epitaxial growth processes which have been studied are described in a number of patents including R. Ruehrwein (U.S. Pat. No. 3,496,024), G. Manley et al. (U.S. Pat. No. 3,619,282), D. Carpenter et al. (U.S. Pat. No. 3,619,283), R. Lee et al. (U.S. Pat. No. 3,642,529), and R. Hager et al. (U.S. Pat. No. 3,725,135).
Another epitaxial growth technique which has been investigated is liquid phase epitaxy ("LPE"). This technique is described in R. Maciolek et al. (U.S. Pat. No. 3,902,924). Liquid phase epitaxial growth offers a number of advantages over both vapor phase epitaxial growth and bulk growth of (Hg,Cd)Te.
One characteristic of epitaxial film grown by both vapor phase epitaxy and liquid phase epitaxy is a tendency to exhibit a compositional gradient along the crystal growth direction. This is particularly true when CdTe is used as the substrate material. Examples of compositional profiles through the thickness of epitaxially grown films are shown in FIGS. 3, 5, 6 and 9 of the previously mentioned Hager et al. patent (U.S. Pat. No. 3,725,135) and in FIGS. 4a-4e of the previously mentioned Maciolek et al. patent (U.S. Pat. No. 3,902,924). The device formed by epitaxial growth may be considered, therefore, to have three regions: the substrate, a graded composition or graded bandgap region, and the epitaxial layer of desired composition.